1. Field of the Invention
This invention directs itself to communication systems for transmitting data to mobile units in a fading channel environment. In particular, this invention directs itself to a communication system utilizing a subcarrier within a commercial FM channel bandwidth utilizing in combination forward error correction coding, interleaving, and the transmission of channel state information. More in particular, this invention is directed to a communication system wherein the encoded digital data is divided into multiple frames, each frame being subdivided into a plurality of data subframes and a synchronization subframe, each subframe including a plurality of bits for establishing the reliability of the transmission channel at the mobile receiver. Further, this invention directs itself to a communication system wherein the receiver correlates received channel state bits with a sequence of bits known to have been transmitted, transforms the correlation into a metric, interpolates across, each data field to form a prequantized data reliability factor, and quantizes the interpolated factor to form a data reliability factor for each bit of data, the reliability factor for each bit being used by a Viterbi algorithm decoder subsequent to deinterleaving of the data.
2. Prior Art
Communication systems for transmitting traffic information and systems for communication in a fading channel environment are known in the art. The best prior art known to the Applicants includes the European Radio Data System (RDS) which is being deployed in Europe for dissemination of traffic-related information to motorists utilizing the existing FM radio broadcasting infrastructure. However, the RDS system is subject to serious drawbacks, such that it is Capable of running at approximately a 0.3 kilobits per second user data rate, as opposed to the instant invention which operates at an approximately 8 kilobits per second user data rate. Further, the RDS system requires the use of coding of the traffic messages to obtain an increase in apparent capacity, in addition to error correction coding, which increases equipment cost, yet still does not attain the data rate of the instant invention.
Two other traffic information systems have been recently introduced in Japan, one utilizing a subcarrier system having an 8 kilobit per second user data rate, but incorporating dynamic control of the subcarrier's injection level. The incorporation of dynamic control of the subcarrier's injection level requires more sophisticated and expensive equipment at the transmitter-end of the system and is more prone to poor performance under weak broadcast signal strength conditions. The other Japanese system developed by SEIKO TELECOMMUNICATIONS, also runs at a high data rate, but is based on a low-robustness FSK modulation, with a wide emission bandwidth. Thus, in application of this system, six to seven different FM stations are required to serve an area where only one FM station is required for the system of the instant invention.
The use of channel state information for compensation in fading channel environments is known in the art. The best prior art in this area known to the Applicants, is U.S. Pat. Nos. 4,945,549; 4,519,068; Hagenauer, J., et al., "Forward Error Correction Coding for Fading Compensation in Mobile Satellite Channels", IEEE Journal on Selected Areas in Communications, Vol. SAC-5, No. 2, February 1987, pp. 215-225; and, Hagenauer, J., "Viterbi Decoding of Convolutional Codes for Fading and Burst Channels," Proceedings 1980 Zurich Seminar Digital Communications, IEEE Catalog No. 80CH 1521-4 Com, pp. G2.1-G2.7. In each of the prior art references, channel state information is utilized by the decoder for soft decision making. However, such systems utilize the channel state information as an estimate of the channel state for a block of data bits, as opposed to establishing a data reliability factor for each bit. In systems such as that disclosed by U.S. Pat. No. 4,945,549, the channel state information is obtained from the power in the recovered pilot tones transmitted. Therefore, a particular sequence of bits is not added to the data stream to both precede and follow each of the data fields, whereby a data reliability factor can be established for every data bit received, as in the instant invention.